For Dave the Gnome

Usage Note: Scottish is the full, original form of the adjective. Scots is an old Scottish variant. Scotch is an English contraction of Scottish that came into use in Scotland as well for a time (as in Burns's "O thou, my Muse! guid auld Scotch drink!") but subsequently fell into disfavor there. In the interest of civility, forms involving Scotch are best avoided in reference to people; designations formed with Scots are most common (Scot, Scotsman, or Scotswoman), but those involving the full form Scottish are sometimes found in more formal contexts. Scotch-Irish is the most commonly used term for the descendants of Scots who migrated to North America, but lately Scots-Irish has begun to gain currency among those who know that Scotch is considered offensive in Scotland. There is, however, no sure rule for referring to things, since the history of variation in the use of these words has left many expressions in which the choice is fixed, such as Scotch broth, Scotch whisky, Scottish rite, and Scots Guards.

http://www.tfd.com/scotch